Interpret a soil-test report

Soil tests are one of the most valuable services you can offer a customer. They allow you to care for turf and landscapes more scientifically, which will increase customer satisfaction. However, tests also can be a selling tool that increases sales and profits. Most professionals who use soil tests charge for this service. Therefore, it can be a no-cost addition to your business.

Soil tests provide information that allows you to develop improved fertility programs that, in turn, help turf and ornamentals in a multitude of ways, including: * Thicker, more even turf * Better foliage color * Improved drought resistance * Improved resistance to disease and insect stresses * Reduced potential for nutrient runoff.

To obtain the maximum benefit from a soil test, you must understand the soil-test report. Your customers will expect you to explain the test results to their satisfaction and use them to develop a reliable fertilizer and amendment program.

The following is an explanation of the standard components of a typical soil-test report. Each numbered section of the sample soil-test report on page 28 corresponds to the sections that follow.

1. Cation exchange capacity (CEC) Nutrients available for plant use (that is, they are in solution) have an electrical charge. In this condition they are referred to as ions. Ions with a positive (+) charge are cations, while those with a negative (-) charge are anions. The soil itself also is electrically charged. The principle source of soil charge is the clay and organic matter it contains. Particles of these two soil constituents have many negative charges on their surfaces. As you might expect, the negative sites on the soil particles attract and hold cations and repel anions.

A soil's cation exchange capacity (CEC) is a measure of its capacity-derived from its negatively charged sites-to hold the exchangeable cations. These include several essential nutrients such as calcium (Ca++), magnesium (Mg++), potassium (K+), copper (Cu++), manganese (Mn++), zinc (Zn++) and iron (Fe++). Think of soil as a "sponge" for nutrients. Higher CECs mean the "sponge" is larger and will hold more nutrients. This requires larger fertilizer applications to fill the "sponge," while lower CECs require smaller but more frequent applications to keep the "sponge" filled.

Most nutrient anions in their available form, such as nitrate (NO3-), chloride (Cl-), sulfate (SO4-2) and borate (BO3-3), are repelled by the soil-exchange complex and, therefore, will easily leach with excess water. (Phosphorus in its available form also is an anion. However, it behaves differently in the soil and does not leach easily.)

Because clay and organic matter attract cations, soils with plentiful amounts of these constituents have higher CECs than sandy soils (which are usually low in organic matter, as well). Even moderate amounts of organic matter (which also benefit soil in other ways, such as improving tilth) provide a great deal of CEC.

2. Soil pH Soil pH (or reaction, as some call it) is the most important chemical factor in the soil. It is the foundation and controlling factor in nearly all important chemical, and many biological, soil reactions. The term pH is used to express the acidity or alkalinity of the soil. It is measured on a scale of 0 to 14, with 7.0 being neutral. Values below 7.0 are acidic, and values above 7.0 are alkaline.

The availability of most nutrients is highest when pH is around 6.2 to 6.8. When the pH is higher, the availability of several nutrients, including phosphorus (P), iron, manganese, boron (B), copper and zinc will decrease. Likewise, as the soil pH decreases below this range, certain nutrients become less available, especially phosphorus, potassium, calcium and magnesium. Further, in strongly acidic soils, some micronutrients, such as manganese, and some non-nutrients, such as aluminum, become toxic to most plants.

Most turfgrasses, flowers, woody ornamentals, fruits and vegetables grow best in slightly acidic soils of pH 6.1 to 6.9. However, some plants have evolved in acid soils and require a lower soil pH (4.9 to 5.5) for best growth. These include rhododendrons, azaleas, pieris, mountain laurel, blueberries, cranberries and some wildflowers and conifers.

You can raise soil pH by adding lime (CaCO3) or lower it by adding elemental sulfur (S) or aluminum sulfate [Al2(SO4)3]. But do not add these materials to your soil to change the pH unless soil-test recommendations indicate you need such amendments.

Do not expect amendments to create a long-term change in pH-most soils tend to return to their "native" pH over time. Changing and maintaining a large area to a pH significantly different from the native pH can require a lot of material and time. Therefore, it usually is wiser to select plants adapted to the existing soil than to attempt to change the soil dramatically. Smaller planting beds for annual flowers and vegetables, by contrast, are easier to manage with regular additions of amendments to adjust the soil pH.

3. Lime Index Some private laboratories and universities use the term Lime Index. Others refer to Buffer pH. Regardless, this test determines the amount of lime needed to correct an acid soil. In this test, the lab adds a prescribed amount of buffer solution to the soil sample. This quickly increases the pH of the soil. The pH is then "read" a second time after the reaction is complete. This new, buffered pH is reported as the Lime Index (LI) or the Buffer pH (BpH). Reports usually omit the decimal point when reporting the Lime Index to avoid confusion with the original soil pH.

A lime recommendation is based on a calculation of the difference between the soil pH and the Lime Index. A higher index indicates that the soil is more susceptible to changes in pH. Typically, a sandy soil will have a much higher Lime Index than a clay soil at the same initial pH because sand has less buffering capacity than clay. This indicates that a given amount of lime will increase the soil pH of the sand much more than the clay. Therefore, a sandy soil will normally receive a relatively low lime recommendation. The actual amount of lime a report will recommend depends on the beginning soil pH and the Lime Index.

4. Organic Matter (OM) Soil organic matter (OM) performs many beneficial functions in soil. It provides nutrients, holds water and improves soil porosity by preventing clay particles from sticking to each other. "Organic matter" to a soil scientist and as shown in soil-test reports does not actually include all organic material in the soil. It refers only to the stable, highly decomposed, tar-like organic material that gives soils a dark color. (However, soil does not have to be dark to have a significant amount of OM.) Soils in more northern climates tend to have higher OM levels because cooler climates permit more OM to accumulate.

When you incorporate organic "residues" into soil, they eventually decompose, leaving relatively small amounts of stable OM behind. While there may be some short-term benefit to soil structure, producing a more permanent increase in soil OM over a large area often is impractical. For example, if you consider that 1,000 square feet of soil 7 inches deep weighs about 23 tons, then a 1-percent change in OM requires about 460 pounds of this dark, highly decomposed OM. To get this much stable OM would require you to incorporate many tons of fresh organic residue.

Increasing OM in small areas, such as annual beds or vegetable gardens, is more practical because incorporating high proportions of organic materials is relatively easy.

Not all labs include OM as part of a standard soil test. However, it can be useful to know this soil property. For instance, soils unusually high in OM can bind certain pesticides and render them less effective. Products vulnerable to being tied up with OM will list this on their labels.

Even "rich" soils typically contain much less than 10 percent OM. One or 2 percent is typical in many areas, particularly arid regions.

5. Available P, K, Ca and Mg Phosphorus, potassium, calcium and magnesium are important nutrients that plants require in relatively large amounts for optimal growth. Reports express soil levels for most of these nutrients in pounds per 1,000 square feet.

Available P, strictly speaking, refers to the amount of this element readily soluble in a weak acid (which means it should be available to plants during the growing season). Exchangeable potassium, calcium and magnesium refer to the amounts of these elements held on the exchange complex in the soil. Elements held on the exchange complex are readily available for use by plants.

As with the sample report we show here, labs sometimes show available nutrients graphically. Depicting nutrients this way can be more useful than a simple number, because other soil properties (such as CEC) may, at least to a limited extent, affect the nutrient levels that are considered "good," "medium" and so on.

Some labs will list nutrient levels in pounds per acre rather than pounds per 1,000 square feet. To convert, divide pounds per acre by 43.56 (1/1,000 acre).

The availability and utilization by plants of the various nutrient elements depends on many factors, such as soil moisture and temperature, the general health and vigor of the plants, interactions and balance between various nutrients at a given soil pH and many other factors. A soil test can identify some of these factors and use them to predict ultimate nutrient availability for plants. Another approach is to use Plant Analysis (tissue sampling) to ascertain what is actually getting into the plant.

6. Base Saturation Base saturation is the percentage of total CEC (negatively charged sites in the soils) occupied by the cations potassium, calcium and magnesium. The ease with which plant roots can absorb cation nutrients increases with the degree of base saturation. In slightly acidic to neutral soils, calcium and magnesium account for up to 80 percent of the exchangeable cations, while potassium accounts for only a small percentage. The box below shows acceptable saturation ranges. Readings lower than these might result in somewhat higher application recommendations.

7. Recommendations Taking all the tested soil properties into account, reports conclude with recommendations for nutrients and amendments. Recommendations on most soil-test reports are fairly self-explanatory and are typically expressed in pounds per 1,000 square feet.

Labs have several purposes in mind when they make their recommendations. The first is to satisfy immediate plant nutritional requirements. The second is to build soil-nutrient reserves to optimal levels. Finally, recommendations are made to reduce over-abundant nutrients.

Liming products are used primarily to raise soil pH, but they also supply calcium. When an analysis indicates a need for magnesium, a dolomitic product (a liming material containing about 10 percent magnesium) is preferred.

William Scott Anderson, CPAg, and Charles Robinson, CPAg, are agronomists for Spectrum Analytic Inc., a soil-analytic laboratory based in Washington Court House, Ohio.

Normally, nitrogen (N) is not part of a soil test. There are several reasons for this. Nitrogen is highly mobile in soil, and a soil sample often does not represent the entire amount of available N that may be in the soil. Conversely, in excessively wet conditions, some of the N that the test indicates is available may be lost before plants can use it. These difficulties have led most labs and other professionals not to depend on routine soil-N tests.

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